Many electronic devices require two or more isolated, stable and precisely regulated supply voltages. For example, microprocessors require a precise supply voltage of 3.3V or less, together with a conventional supply voltage of 5V. As these devices reach ever-smaller dimensions, they require increasingly higher power and higher operating efficiency. The trend of reducing component dimensions, combined with the trend of providing increasing numbers of precision supply voltages, leads to considerable difficulties in selecting suitable low cost circuits for a given purpose that can provide high energy-conversion efficiency and which have a relatively simple construction.
A typical prior art switching power supply configuration is illustratively shown in FIG. 1. A primary circuit 10 including at least one switch controlled by a pulse-width modulated square wave signal is connected to a plurality of loads LD1, LD2, LD3 via respective secondary circuits 12. The secondary circuits 12 may be buck converters, for example, which are well known to those skilled in the art.
The main supply output with respect to the load LD1 is regulated by a feedback path to the primary 10 which includes a PWM control circuit 14 for modulating the pulse width of the switch control signal at a fixed frequency. The control circuit 14 regulates the output voltage VO1 by countering variations in the load LD1 or in the input voltage Vin.
A variation in the input voltage is compensated by the effect of the PWM regulation on all of the outputs. In contrast, a variation in the load LD2, LD3, which causes alterations in the output voltages VO2 and VO3 cannot be taken into account by the PWM control circuit 14, since the two outputs are in an open-loop configuration.
To regulate the output voltages VO2 and VO3 when there are variations in the load, a regulator circuit 16 is arranged in cascade for each output voltage. Prior art approaches for constructing regulator circuits in cascade include linear regulators, cascaded DC/DC converters, and magnetic amplifiers.
A linear regulator is relatively simple, inexpensive, and easy to design. One significant disadvantage thereof is its low efficiency. For this reason, linear regulators are primarily used exclusively in low current applications.
DC/DC converters arranged in cascade with the output have performance advantages in terms of efficiency, voltage regulation, and permissible current. However, they are disadvantageous from a cost standpoint since a DC/DC converter requires the use of power switches, inductors, capacitors, and control circuits. Moreover, the introduction of such a converter gives rise to additional noise and produces a ripple in the output current. This ripple has to be corrected by filters, or by synchronizing the regulator with the main PWM control circuit.
Magnetic amplifiers can be described as regulators in cascade with programmable delay switches and are the most common choice for medium and high-power cascade regulators. The main component of such regulators is a reactive component which can be saturated to act as a magnetic switch, since it has high impedance when non-conductive and low impedance when saturated.
The magnetic amplifier achieves a desired control and regulation function and is a relatively simple circuit. It also provides safe performance with large loads. Yet, with a low load or no load, on the other hand; regulation is less efficient. Further disadvantages are that it provides limited switching frequency and may be rather large or bulky to implement.
An effective approach within a wide power range for multi-output switching power supplies and, in particular, in medium or high-power applications, is the cascaded PWM regulator. This regulator includes an auxiliary switching device (generally, but not exclusively, a MOSFET power transistor) controlled by a PWM signal. The PWM signal is generated by a control circuit synchronized with the main PWM control circuit 14.
The regulator preferably works by time modulation of the leading edge of the PWM control signal over time. The auxiliary switch blocks propagation of the voltage signal established in the secondary winding to the output of the power supply. The control circuit of the cascade regulator synchronizes the conduction periods of the main circuit switch and the auxiliary switch with the trailing edges of the respective square-wave control signals.
The advantages of this regulator over magnetic amplifier regulators are lower cost, smaller dimensions greater reliability, and better performance. The circuits required to control the switch may be complex, but they may be integrated in a single chip which helps to offset this complexity.
The operation of a control circuit for a cascaded PWM regulator is similar to that of a voltage converter/reducer (i.e., a buck converter). That is, it controls a switch device that is suitable for blocking an input voltage for a predetermined period of time, producing at the output a duty cycle less than that present at the input. The voltage value at the output consequently depends on the feedback loop of the control circuit, which controls the conduction or non-conduction of the switch.
Examples of PWM regulator circuits connected in cascade are described, for example, in U.S. Pat. Nos. 6,130,828 and 6,222,747. The '828 patent, which is assigned to Lucent Technologies, relates to a multi-output converter with self-synchronized pulse-width modulated regulation. The PWM control signal of the auxiliary switch associated with the regulator is generated by direct control of a driver circuit with input hysteresis by a ramp signal. This signal is generated by an integrator circuit disposed downstream from a circuit for amplifying the voltage error present at the regulated output. The amplitude of the signal thus depends on the output voltage error to be compensated by the regulator.
The '747 patent, which is assigned to Artesyn Technologies, relates to a control circuit connected in cascade in a multi-output switching power supply. The circuit described therein includes a synchronous pulse generator arranged to detect the trailing edge of the voltage established in the secondary transformer winding. A ramp signal generator is controlled by the pulses emitted from the pulse generator. The ramp signal generator is connected to the non-inverting input of a comparator, the inverting input of which receives a signal that is indicative of the output-voltage error. This signal is emitted by an error amplifier circuit that can compare the voltage at the output of the regulator with a predetermined internal reference voltage.
The ramp signal is reset and triggered at the same moment in time, which coincides with the trailing edge of the voltage in the secondary transformer winding. A theoretical duty cycle to be achieved in controlling the auxiliary switching device is 100%. That is, an operative condition in which the switching device is always conducting is envisaged.